Special problems are presented by the injection molding of elongated, slender workpieces, i.e. hollow items such as vials, test tubes, pipettes and the like, where the ratio of the inside diameter to the length is greater than 2:1. The core which forms the inside of these articles is usually supported at its base by a core plate (and an ejector plate) and typically extends cantilever fashion into the cavity defining the outer surface of the workpiece to be molded.
In cases where the molded workpiece has an opening on both ends (for example, a pipette or an open-ended tube), the small end of the core is typically supported on the cavity wall. However, especially in the case of a pipette, the cross-section of the core supported by the cavity wall is quite small, and therefore the core is usually too weak to offer any substantial resistance to the high injection pressure required to fill the mold cavity in the course of injection. As a result, the core will tend to be laterally displaced.
During the injection process, the forces tending to dislocate the core from its central position coaxial with the cavity depend upon the injection pressures used. The thinner the wall of the article, the higher are the pressures which are required to fill the cavity in a desired minimum molding cycle. Even under ideal flow conditions, with the filling taking place from the center of the cavity (opposite the core tip), a very slight uneven flow of plastic into the gap between the cavity wall and the core will often result and will start to deflect the core to one side. This increases the uneven condition and results in an even greater unbalance in the flow. This procedure may ultimately push the core completely toward and against the wall of the cavity, and thereby create an unusable workpiece. While the latter example may represent an extreme case, it is true that some coreshift takes place in all such molds. Vial molds can be filled, with fairly good success, with use of a central gate. Under ideal operating conditions, the coreshift can be held down to an acceptable range of uneven wall thicknesses compatible with quality requirements. Tubular workpieces such as pipettes cannot be center-gated for obvious reasons. Any eccentric gating would cause the plastic flow to generate very large side forces on the core and thus create excessive coreshift.
In cases of this kind, the workpieces can be side-gated near the open end of the tube (i.e. near the support of the core, preferably with two or more gates evenly spaced around the circumference). This method is now commonly used for pipettes, hypodermics, and other articles with a small opening in the top end, and also for test tubes with a slenderness ratio of more than 3:1. In this process, the plastic entering the cavity acts on the core near its base, where it is well supported, and then flows up toward the tip. The plastic flow forms a supporting wall between the core and the cavity wall, helping to sustain the core centrally. The coreshift is thereby held to a minimum. The disadvantage of this method is the need for a cold-runner system, i.e. a system of plastic-flow channels which must be filled with every shot (injection cycle) to bring the plastic to the cavity. This requires an injection capacity in excess of that which would be required if only the volume defining the workpieces had to be filled. It also requires higher pressures to make up for the flow losses in such a system. Worst of all, considerably more injection-cycle time is needed to permit the runner system to cool before ejection. Typically, a workpiece would be ready to eject in about 6 to 8 seconds, whereas an adequately sized runner would need at least 15 to 20 seconds to be solid enough for ejection. Such runners, after ejection, can be reground and reused, but in some cases they must be scrapped on account of the deterioration of the material during the injection cycle which may impair the physical properties of the material. Another disadvantage of the gating at the base of the core is that the plastic, as it flows toward the tip, will compress the air present inside the cavity. Careful venting is required to release this trapped air, as otherwise the cavity space defining the workpieces will not be filled completely at the tip of the core. This venting, however, sometimes presents considerable difficulties and also leaves marks on the workpieces.
By comparison, in systems which permit injection near the tip, "hot-runner" molding is possible, i.e. the runner is never cooled down. The gate, which is the orifice by which the plastic enters the cavity, in some instances freezes sufficiently during mold-open time to prevent leaking of plastic and will be opened again by the injection pressure during the next cycle; alternatively, a mechanical valve stops the flow after every injection cycle (valve gate). Such a hot-runner molding system has the following advantages over the previously described method:
(1) Only the workpieces are molded but not the runners. PA1 (2) A higher molding speed is permitted. The cycle is determined only by the wall cross-section of the molded workpiece and the efficiency of mold cooling. PA1 (3) No regrinding is needed and no material losses are encountered. PA1 (4) There are no venting problems.
One notable attempt has been made to solve this difficulty by surrounding the core with a sleeve which reaches far up into the cavity near the gate at the tip of the workpiece. The sleeve is guided by both the cavity wall and the core and, during injection, is pushed toward the open end of the molded workpiece by the molten plastic entering the cavity. While this system would appear to provide a solution for the problem of core support during injection, the technical difficulties associated therewith are very serious and, as far as is presently known to me, have never been solved.
It has also been proposed to steady the core near its tip by a spring-loaded support which holds the core centered until the plastic pressure pushes the support out of the way.
The disadvantages associated with the last-mentioned solution relate to the fact that the forces available are very small and give only marginal support. Moreover, any device with sliding parts and springs inside the mold cavity is subject to wear, sticking and/or leaking. Such a process would also result in the marking of the molded workpieces at the location of the supports.
U.S. Pat. Nos. 3,301,928 and 4,128,381 are representative of the prior-art solutions described above.